1. Field of the Invention
The present invention relates to III-nitride semiconductor devices, and more particularly to III-nitride semiconductor devices with an improved p-contact structure within these devices.
2. Description of the Related Art
III-nitride family is a group of direct-bandgap compound semiconductors composed of group III-A elements in periodic table and nitrogen. This group of material includes binary compounds such as aluminum nitride (AlN), gallium nitride (GaN), indium nitride (InN), ternary compounds such as AlGaN, InGaN, InAlN, as well as quaternary compounds AlxInyGa1-yN. III-nitrides have attracted a lot of attention lately due to wide coverage of energy gaps (Eg), ranging from 0.8 eV (InN) to 6.2 eV (AlN). Light emitting devices made from III-nitride material can therefore emit light with wavelength covering the whole visible spectrum. Nitride-based green, blue and ultra-violet (UV) light emitting diodes are now commercially available and are applied to applications such as displays, indicator lights, traffic lights and even illumination light sources. Nitride-based laser diodes (LDs) were also commercialized and utilized in new generation digital video disk (DVD) system with much higher storage capacity. In addition, with high mechanical and temperature stabilities, III-nitride materials are very suitable for fabricating high power electronic devices. The supreme material properties have made this material system an attractive candidate for future optoelectronics devices.
In almost all semiconductor devices, low-resistance ohmic contacts are required for optimum device performance. High-resistance contact degrades device performance during operation due to excessive heat generated at the contact interface with the semi-conducting material. This issue is particularly important to III-nitride materials since the large energy bandgaps make it difficult to obtain as good ohmic contacts as in other III-V compounds with smaller bandgaps such as GaAs and InP. Thus, fabricating low-resistance ohmic contacts has been one of the major research topics in III-nitride based electronic and optoelectronic devices. In the past few years, progress has been made with good ohmic contacts achieved on both n- and P-type GaN and AlGaN. Ohmic metal contacts to N-type III-nitrides can be obtained using Ti/Al etc. However, due to the large acceptor (mainly Mg) activation energy and the lack of metals with large enough work function, only limited success has been reported for P-type III-nitrides. So far, Ni/Au, Pd/Au and Ag have demonstrated acceptable ohmic contact to Mg doped P-type III-nitride materials. However, to achieve higher device performance, an even more improved P-type contact is still needed.
III-nitride materials are known to exhibit strong polarization effect. Strong piezoelectric polarization often exists in layered structure. By taking advantage of this effect, a novel approach to form good metal-semiconductor ohmic contacts was realized. With appropriate polarization, the thickness of the Schottky barrier at the metal-semiconductor interface can be decreased, and therefore increasing the carrier tunneling probability. Such polarization-enhanced ohmic contacts to P-type III-nitride can be made by depositing a compressively strained capping layer on a relaxed buffer layer. Examples of such structures are a strained thin GaN layer deposited on the relaxed thick AlGaN layer or a strained thin InGaN layer deposited on the relaxed thick GaN layer.
In reality, however, it is not easy to fabricate such structures, especially for depositing InGaN layer on GaN layer due to the large lattice mismatch between InGaN and GaN.
Additionally, if the bandgap of the InGaN layer is smaller than the bandgap of the light generation layer of a light emitting device, the InGaN layer can be light absorbing and reduces the light output efficiency of the device. In the case of short wavelength light emitting diode in the 400 nm range, the large absorption coefficient of InGaN material at this wavelength can make the light emitting device utilizing the InGaN capping layer very inefficient.
Accordingly, it is an intention to provide a III-nitride semiconductor device with an improved P-type contact structure to overcome the above drawbacks encountered in the prior III-nitride semiconductor devices.